High-fin integral finned tube of heat-resisting alloys, and multi-pass process for making the same

ABSTRACT

Rolling of high-fin integral finned tube from stainless steel or other difficult to form material, in two operations with an annealing step between rolling operations. Each rolling operation quickly forms narrow helical grooves to displace metal into wide helical ribs or rib portions, followed by more gradual widening and deepening of the grooves with additional metal displacement, and lateral application of pressure to the ribs to form it to required fin shape.

United States Patent 1 Laing et al.

.1111 3,795,125 1 Mar.5, 1974 1 HIGH-FIN INTEGRAL FINNED TUBE OFHEAT-RESISTING ALLOYS, AND MULTI-PASS PROCESS FOR MAKING THE SAME [75]Inventors: David D. Laing, Plymouth; John W.

issott, Dearborn Heights, both of Mich.

[731 Assignee: Universal Oil Products Company, Des Plaines, Ill.

22 Filed: Jan. 27, 1972 211 Appl. No.: 221,358

3,383,893 5/1968 Counts 72/98 3,174,319 3/1965 Sayukoyama et al... 72/963,379,042 4/1968 Brown 1 l 72/78 3,648,502 3/1972 Klug et al. 72/78Primary ExaminerLowell A. Larson Attorney, Agent, or Firm-Whittemore,Hulbert &

Belknap [57] ABSTRACT Rolling of high-fin integral finned tube fromstainless steel or other difficult to form material, in two operationswith an annealing step between rolling operations. Each rollingoperation quickly forms narrow helical grooves to displace metal intowide helical ribs or rib portions, followed by more gradual widening anddeepening of the grooves with additional metal displacement, and lateralapplication of pressure to the ribs to form it to required lin shape. 3I

11 Claims, 6 Drawing Figures l/VS/DE T086 SURFACE PATENTED 51974 FIGIl/vsnna Tuag mF/vce FIG.5

HIGH-FIN INTEGRAL FINNED TUBE OF HEAT-RESISTING ALLOYS, AND MULTI-PASSPROCESS FOR MAKING THE SAME BRIEF SUMMARY OF THE INVENTION The rollingup of material from the outer wall of a tube by effecting relativerevolution of a plurality of sets of axially aligned finning discs,while rotating the discs, is well known. When working with malleablematerials such as copper, particularly where relative high fins are notrequired, present techniques are satisfactory.

However, when working with metals or alloys which are difficult to work,particularly when work hardening occurs as in stainless steel, it hasheretofore been impossible to attain the required results. The problemis made more difficult where the fin is to be of substantial thicknessthroughout and thus to have a relatively blunt outer edge as compared tofins the outer edge of which are substantially knife edged.

An example of finned tubing of this type is designed for use in coolingliquid sodium by air, as is done in nuclear reactor applications, morespecifically fast breeder reactors. finned tube is produced by two ormore rolling operations with annealing of the tube intermediatesuccessive rolling operations. Successive rolling operations aresubstantially similar, except that in subsequent operations, the leadingdiscsoperate in the helical grooves left by the preceding operation.

In each operation a set comprising a multiplicity of discs are assembledin engagement in axial alignment and contact the tube with the axescrossed in space to conform to helical fin formation to the assembly ofcircular discs. The first few discs of each set have the edges thereoftransversely curved on a relatively short radius to facilitatepenetration into the material of the tube, and successive discs increasein diameter by relatively large amounts. This quickly forms one or morrelatively deep, relatively narrow helical grooves, and displaces asubstantial quantity of metal into a relatively wide, relatively lowhelical rib in the first operation and into the base or root portions ofpreviously formed and shaped ribs in subsequent operations. Thereafterthe successive discs are shaped to hav gradually wider edges, and haverelatively less increase in diameter from disc to disc, so as to producea gradual widening and deepening ofthe groove, with consequentdisplacement of more metal into the eventual fin. In addition, theangularity of the sides of the discs from radial surfaces decreasesgradually to a very low value, so that fins are formed having athickness at their tips amounting to a very substantial percentage ofthe fin thickness adjacent the base.

The successive rolling operation or operations ar carried out with setsof discs of increased thickness or pitch, as compared to the pitch ofthe discs of the immediately preceding set, thus producing helicalformations on the tube of longer lead.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary sectionthrough a finished finned tube produced in accordance with the presentinvention.

FIG. 2 is a generalized outline of a single fin produced in accordancewith the present invention.

FIG. 3 is a sectional view similar to FIG. I of the same tube followingits initial rolling' operation with partially formed fins thereon.

FIG. 4 is an enlarged diagrammatic view with wall sections of FIGS. 1and 2 superimposed.

FIG. 5 is a fragmentary outline view of successive disc edge portions ofa set of discs.

FIG. 6 is a fragmentary outline of a finning disc to show dimensionsdefined in the specification.

DETAILED DESCRIPTION Referring first to FIG. 1, there is shown anenlarged fragmentary cross-section of a finished tube 10 produced inaccordance-with the present invention. Very approximately, the criticaldimensions of this tube are as follows:

DIMENSIONS INCHES Fin height l2 (H30 Fin thickness at tip l4 0027 Finthickness at root I6 004] Fin thickness, average l8 0.034 Fin space,average 20 0.058

Wall thickness 22 0.107

From the foregoing tabulation it will be noted that with an exceptionalfin height of 0.130 inches, the fin thickness at the tip is over 20percent of fin height, and is over 65.percent of the fin thickness nearthe root.

A more generalized fin cross section outline of a single fin is seen inFIG. 2, where dimensions are applied to show fin thickness at tip andbase, fin height and wall thickness.

' From the foregoing it is apparent that the fin of the presentinvention differs from conventional fins in its exceptional thicknessadjacent its tip, and its height as compared to its average thickness.In general terms the present invention may be considered as producingfins having a minimum height of 0.100 inches, a thickness adjacent itstip which is not less than 20 percent of the fin height, a thicknessadjacent is base or root which is not less than 25 percent of the finheight, and a thickness at the tip which is not less than 60 percent ofits thickness adjacent its base. This represents a fin crosssectionwhich is characterized by substantial tip thickness and which isadvantageous in rendering the entire fin surface useful in heattransfer, as well as in providing a substantial increase in area.

In the production of this new finned tubing, mention has been made of aplurality of successive rolling operations with an annealing stepintermediate successive rolling operations. In FIG. 3 there is shown thecrosssectional shape of the tube wall with ribs or partially formed finsfollowing the first rolling operation, where this operation is designedto be the first of two, the second of which produces the tubeconfiguration as seen in FIG. I. In FIG. 3, the partially rolled tube 26has the approximate dimensions tabulated below:

DIMENSION INCHES Rib height 28 0.085 Rib thickness at tip 30 0.032 Ribthickness at root 32 0.039 Rib thickness, average 34 0.035 Rib space,average 36 0.054 Wall thickness (H29 Here it will be observed that theaverage rib thickness is about 40 percent of the rib height, and thethickness at the tip is over percent of the thickness near the fin root.

In FIG. 4 theoutline of the cross-sectional shapes of the finished tubeis superimposed over the outline of the cross-section of thesemi-finished tube. The inside tube diameter at 40 is used as the commonreference line, and the rib or partially formed fin 42 seen at the leftof the Figure is superimposed so far as lateral position is concernedover the finished fin configuration 44. By comparing these Figures, itwill be noted that the groove or channel between fins has been deepenedby the amount shown at 46, while the crest or tip of the fin has beendisplaced radially outward by the amount shown at 48. At the same timeit will be noted that the outline of the entire upper portion of thepartially formed fin'or rib conforms very closely with the mid portionof the finished fin, as indicated by the matching side surfaces seengenerally at 50.

FIG. 4 also graphically illustrates a very important feature of thepresent invention and that is the increase in pitch or axial spacing ofadjacent fin or rib convolutions as produced by the second rollingoperation as compared to the first. In the specific example illustratedin FIGS. 1, 3 and 4 the pitch or axial spacing of the roll discs whichproduced the semirolled tube of FIG. 3 was 0.082 inches, while that ofthe discs which produced the finished finned tube of FIG. 1 was 0.086inches. This represents an increase in pitch or lead of nearly 5percent. This of course requires that in multiple pass rolling, thepitch produced by the first pass must be sufficiently smaller than thefinal desired pitch as to produce the required number of fins per inch,in this case, eleven. Referring now to FIGS. 5 and 6, there is anenlarged diagrammatic showing of the edge of an assembly or set of discswhich are separately designated 56a to 56 An important feature of thepresent invention is that the profiles, or cross-sectional shape of theside surfaces of the edge portions of the corresponding discs issubstantially the same in the two or more setsor assemblies. The onlysignificant difference in correspoinding discs is the thickness thereofas indicated by the dimension 58, which as previously described, isincreased in successively acting sets of discs.

In FIG. 6 there is shown in cross-sectional outline the generalizedshape of the periphery of a fin-forming disc. In this Figure the sidesof the edge portion of the disc a re shown at 60 and are substantiallystraight in radial cross-section of the disc. In other words, thestraight line side elements together define a frustoconical surface, theincluded cone of which is determined by the angle A between straightline elements 60 and the line 62, which is parallel to the centralradial plane 64 of the disc. Where the element 60 is inclined at 50 tothe plane 64 (or intersects the line 62 at this angle) the included coneangle is of course 170.

The disc has a periphery 65 which is rounded in cross-section, and itsradius of curvature is designated R in the Figure. Portions 66 of theedge surface smoothly interconnect the rounded edge with the conicalsides. It will be apparent that the smaller the radius R, the morereadily the disc will penetrate into the material of the tube.

The thickness of the disc adjacent the rounded edge portion 65,66 isdesignated 68. This dimension is determined by the distance between theintersections of the line 67, tangent to the tip 65, with the extensionof straight line side elements 60. The thickness of the disc at anannular zone spaced inward of the edge at a constant dimension 69 isdesignated 70.

The progressive change in shapes and dimensions of the discs 56a56q ofcourse determines the specific manner in which the metal of the tube isdisplaced and shaped into final form.

Initially, it isrequired to obtain a quick penetration into the tube, toestablish a relatively narrow, relatively deep groove or channel,adjacent convolutions of which define therebetween a relatively wide andshort rib, themetal of which will eventually be shaped into the outeredge of the fin. Thereafter, by the progression in shapes and dimensionsof successive discs, the

groove or channel is gradually deepened and widened by a gradualincrease in the diameter and tip width and radius of transversecurvature, to move more metal into ribs establishing the eventual finportion, and to apply lateral pressure to shape the material. In generalterms, the first few discs, as for example the first six discs 56a-56fhave a tip thickness dimension 68 which is constant and desirably areabout percent of the tip thickness dimension 68 of the final few discs,as for example the final five discs 56m-56q, with a gradual increase intip thickness in the intermediate discs.

At the same time, as the tip thickness dimension 68 increases, theradius of the rounded periphery 65 in creases, and the radius R of thefirst few discs are desirably about one-half the radius R of the lastfew discs, with a gradual increase from disc to disc in the centrallylocated discs.

The root or gage thickness 70, as measured at a constant gage distance69 is constant for the first few discs, as for example the six discs56a-56f, and in successive discs is decreased gradually to the last fewdiscs, as for example the last five discs 56m-56q, which have a uniformgage dimension desirably about percent of that of the leading discs56a-56f.

It will of course be apparent that the widening of the tip width 68 andthe reduction in gage width 70 results in a reduction in the angle A,which represents the inclination of the conical side surfaces 60 to theradial plane 64. In production of the fin structure specificallydisclosed herein, the angle A is reduced from about 5 throughout thefirst few discs to about l throughout the last few discs, and thisreduction is gradual throughout intermediate discs 56f56m.

The first few discs are arranged to provide a quick penetration into thetube wall, and to accomplish this the disc diameter in the first fewdiscs increase very substantially as compared to the increase indiameter of successive discs thereafter. In addition, the first sets ofdiscs are positioned relative to the tube such that the first disc 56apenetrates to a substantial depth, as for example 0.030-0040 inches,into the metal of the tube.

The next few discs, for example the two discs 56!? and 561' desirablyhave a diameter exceeding the diameter of the preceding disc by from0010-0020 inches.

The following discs show a more gradual increase in diameter, as forexample about one-third of that of the first few discs, and in addition,in the last discs of the series some discs may be identical with anadjacent disc, thus further decreasing the average change in dimensionsto a more gradual condition.

The penetration of the edges of the discs into the material of the tubeand the pressure applied to the tube by the discs has the effect offorming fins, reducing the inside diameter of the tube, reducingeffective wall thickness as measured between fin convolutions, andelongating the tube. In. order to accommodate the tube elongation thataccompanies each rolling operation, the axial spacing of median planesof discs, which is uniform for each set of rolls, is increased insucceeding sets.

It is customary to employ three sets of discs at uniform circumstantialspacing around the tube. Thus the radial components of force actingbetween the rolls and the tube are balanced. However, a pin or mandrelis also provided to control the inside diameter of thefinished tube. Thepin is cylindrical and is smaller by a few thousandths of an inch thanthe inside diameter of the tube as it approaches the sets of finningdiscs. The initial discs, which have narrower edges which are rounded tosmaller diameters than subsequent discs, penetrate the material of thetube without forcing its inner surface into engagement with the pin.Somewhere adjacent the center of each set, radial pressure is sufficientto force the inside surface of the tube into engagement with the pin,and thereafter increase in effective diameter of discs and increase intip width is effective to displace more metal than when the tube insidediameter is being reduced.

It has been mentioned that the profiles of the edge portions ofcorresponding discs in subsequently operat ing sets of discs aresubstantially the same as in preceding sets, differing only inthickness. It will be-understood that alldiscs have flat sides and areassembled in tight side-by-side abutment with each other.

In carryingout the procedure, a tube has its end slightly pointed toprovide smooth engagement with the leading discs of the sets whichperform the initial rolling operation, and the sets are positioned atradial spacings such that when fully engaged beyond the pointed endsection, the leading disc penetrates the tube material to a greaterdepth, for example double the depth to which the next few discs increasethe penetration. In subsequent rolling operations, after annealing, setsof discs are positioned so that the leading disc bottoms in the grooveor channel in the partly rolled tube without substantial penetration.

The annealing step of course depends on the particular material, butwhere the tube is Type 304 stainless steel, the tube is advanced throughan annealing furnace having a muffle length of about feet at a speed of6 inch/min., the furnace being at l,950F. The tube initially had ahardness of R 77 after the initial rolling operation, and afterannealingits hardness was reduced to R ,72.

In general terms, the included cone angle at each side of the discs ineach set increases from a value of about l65l70 to about 178.

An inspection of FIG. 4 shows that in increasing fin height in thesecond operation, the increase in fin outside diameter 48 isapproximately equal to the increase in fin space depth or reduction inwall thickness shown at 46.

In the foregoing it will be noted that the term fin. is reserved for thefinal shape produced by the final rolling operation, and that until thusformed, the material that ultimately becomes part of the fin is referredto as a rib.

In the sets of discs, the disc which initially engages the tube isreferred to as the first or leading disc of the set, and the disc at theopposite end is referred to as the last, final or trailing disc.

What we claim as our invention is:

1. The method of forming fins of at least 0.100 inches fin height and ofpredetermined pitch on tubes of difficult-to-roll metal such asstainless steel which comprises forming partial fins of substantiallyless than final height and of less than said predetermined pitch in afirst operation by supporting the tubes on mandrels and applyingpressure of metal deforming intensity to limited zones radially inwardlyand progressively around the tubes in helical paths of less than saidpredetermined pitch to produce partiallyformed fins, annealing thetubes, and providing a second forming operation by supporting the tubeson a mandrel and applying pressure of metal deforming intensity tolimited zones both at the bottom of the spaces beween partially formedfins and to the sides of the partially formed fins along helical pathsof said predeermined pitch to displace metal from the bottom of thespaces between adjacent fins, and to displace metal laterally of thefins to increase the outside diameter of the tin crests, both metaldisplacements serving to increase the height of the individual fins.

2. The method of claim 1 which comprises in each forming operation bothbefore and after the annealing step, initially applying pressure torelatively narrow zones to form relatively deep narrow grooves leavingrelatively wide ribs formed partially of material of the tubes displacedby formation of the grooves, and thereafter gradually widening the zonesof pressure application and deepening the grooves to displace additionalmaterial into the ribs and to apply pressure to the sides of the ribs toform ribs in the first operation and fins in the second operation ofexceptional height and width adjacent the outer edge portion thereof.

3. The method of claim 2 in which the increase in the pitch of finconvolutions formed by the second operation over the pitch of ribconvolutions formed by the first operation is about 3-5 percent.

4. The method of claim 1, which comprises using a set of axially aligneddiscs in each operation, the discs of each set having the first fewdiscs provided with rounded peripheral portions of a predetermined smallradius in radial cross-section and of a predetermined substantialincrease in diameter from disc to disc whereby to penetrate into thematerial of the tube quickly to substantial depths, the remainder of thediscs of each set having peripheral portions of progressively greaterradius of curvature in cross-section and of sub: stantially lessincrease in diameter from disc to disc, whereby in each rollingoperation a relatively thick rib portion of material is displaced fromthe tube material and thereafter shaped into a narrower highercrosssectional shape.

5. The method of claim 4 in which the side surfaces of each discadjacent its periphery is conical and in which the included cone angleof each side increases from about 165-l at the first few discs toapproach at a reduced rate of penetration into the tube material whileapplying lateral pressure to shape the rib into a tin of the requireddimension.

7. The method of claim 6 which comprises at the start of each ribforming operation, applying pressure to obtain a metal displacingpenetration into the tube material several times as great as thepenetration obtained 7 upon subsequent applications of presusre.

Ti. The method of claim 7 in which the tin height pro duced in the firstoperation is at least 0.060 inches and in the second operation is atleast 0.100 inches.

9. The method of claim 7 in which the fin height produced in the firstoperation is at least 0.075 inches and in the second operation is atleast 0.1 15 inches.

10. The method of claim 6, which comprises the step ofincreasing thepitch of fin convolutions between successive rolling operations.

11. The method as defined in claim 10 in which the increase of pitchbetween successive rolling operations is about 3-5 percent.

1. The method of forming fins of at least 0.100 inches fin height and ofpredetermined pitch on tubes of difficult-to-roll metal such asstainless steel which comprises forming partial fins of substantiallyless than final height and of less than said predetermined pitch in afirst operation by supporting the tubes on mandrels and applyingpressure of metal deforming intensity to limited zones radially inwardlyand progressively around the tubes in helical paths of less than saidpredetermined pitch to produce partially formed fins, annealing thetubes, and providing a second forming operation by supporting the tubeson a mandrel and applying pressure of metal deforming intensity lolimited zones both at the bottom of the spaces beween partially formedfins and to the sides of the partially formed fins along helical pathsof said predeermined pitch to displace metal from the bottom of thespaces between adjacent fins, and to displace metal laterally of thefins to increase the outside diameter of the fin crests, both metaldisplacements serving to increase the height of the individual fins. 2.The method of claim 1 which comprises in each forming operation bothbefore and after the annealing step, initially applying pressure torelatively narrow zones to form relatively deep narrow grooves leavingrelatively wide ribs formed partially of material of the tubes displacedby formation of the grooves, and thereafter gradually widening the zonesof pressure application and deepening the grooves to displace additionalmaterial into the ribs and to apply pressure to the sides of the ribs toform ribs in the first operation aNd fins in the second operation ofexceptional height and width adjacent the outer edge portion thereof. 3.The method of claim 2 in which the increase in the pitch of finconvolutions formed by the second operation over the pitch of ribconvolutions formed by the first operation is about 3-5 percent.
 4. Themethod of claim 1, which comprises using a set of axially aligned discsin each operation, the discs of each set having the first few discsprovided with rounded peripheral portions of a predetermined smallradius in radial cross-section and of a predetermined substantialincrease in diameter from disc to disc whereby to penetrate into thematerial of the tube quickly to substantial depths, the remainder of thediscs of each set having peripheral portions of progressively greaterradius of curvature in cross-section and of substantially less increasein diameter from disc to disc, whereby in each rolling operation arelatively thick rib portion of material is displaced from the tubematerial and thereafter shaped into a narrower higher cross-sectionalshape.
 5. The method of claim 4 in which the side surfaces of each discadjacent its periphery is conical and in which the included cone angleof each side increases from about 165*-170* at the first few discs toapproach 180* at the last few discs.
 6. The method of forming fins onthe exterior of metal tubes in which the fins are characterized byexceptional height and a thickness at the crest which is at least halfthe thickness adjacent the root which comprises repeatedly applyingpressure in a first rolling operation to adjacent narrow zonesprogressively around the tube in a helical path to penetrate into thetube material and to initially displace a substantial quantity of metalinto a helical, short, thick rib and thereafter progressively displacingmaterial from wider zones with reduced penetration while applyinglateral pressure to the rib, terminating the application of pressure andannealing the tube, and thereafter in a second rolling operationrepeatedly applying pressure in the helical spaces between ribconvolutions in narrow zones with increased penetration into the tubematerial to displace a further substantial quantity of material into therib, and finally displacing material by penetrating into the tubematerial over zones of increased width at a reduced rate of penetrationinto the tube material while applying lateral pressure to shape the ribinto a fin of the required dimension.
 7. The method of claim 6 whichcomprises at the start of each rib forming operation, applying pressureto obtain a metal displacing penetration into the tube material severaltimes as great as the penetration obtained upon subsequent applicationsof presusre.
 8. The method of claim 7 in which the finheight produced inthe first operation is at least 0.060 inches and in the second operationis at least 0.100 inches.
 9. The method of claim 7 in which the finheight produced in the first operation is at least 0.075 inches and inthe second operation is at least 0.115 inches.
 10. The method of claim6, which comprises the step of increasing the pitch of fin convolutionsbetween successive rolling operations.
 11. The method as defined inclaim 10 in which the increase of pitch between successive rollingoperations is about 3-5 percent.